From: Rafael J. Wysocki
Sent: 01 April 2021 14:50
...
So what exactly is wrong with using "packed"? It is way easier to understand for a casual reader of the code.
Because it is usually wrong!
If I have: struct foo { u64 val; } __packed;
And then have: u64 bar(struct foo *foo) { return foo->val; }
The on some cpu the compiler has to generate the equivalent of: u8 *x = (void *)&foo->val; return x[0] | x[1] << 8 | x[2] << 16 | x[3] << 24 | x[4] << 32 | x[5] << 40 | x[6] << 48 | x[7] << 56;
If you can guarantee that the structure is 32bit aligned then it can generate the simpler: u32 *x = (void *)&foo->val; return x[0] | x[1] << 32;
(Yes I've missed out the 64-bit casts)
This is why you should almost never use __packed.
There are historic structures with 64 bit items on 4 byte boundaries (and 32 bit values on 2 byte boundaries). Typically most of the fields are shorter so can be read directly (although they might need a byte-swapping load).
David
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